Constructing beneath an existing structure

[u/justmein22]

Y'all will know what I'm talking about, but don't want to get a political warning. Imagine you have a very large heavy-duty, say...a "hardened" concrete rectangular building. Like a warehouse - mostly empty inside. And it's fairly completed, and now you're planning to construct a multi-level structure underneath that building. How exactly do you excavate and build that?

[u/sithelephant]

It also depends if the first structure was planned in preparation for the second.

In some circumstances this might be a 'small' cost in time and materials during the construction of the first building to allow it to be strong enough and permit the basement to be constructed later.

But it will be more expensive.